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In the 16th century, most of Europe was using the Julian calendar, created in the first century B.C. The Julian calendar operated on a four-year-cycle, with 365 days in the standard year and a 366-day year every fourth year.

The 365 1/4-day year came very close to approximating the solar year, the length of time it takes for the Earth to revolve completely around the sun (about 365.2424 days, according to The Galileo Project). However, over the span of centuries, the difference (about 11 minutes, 14 seconds) added up, and the calendar began to become noticeably inaccurate.

The vernal equinox, which typically occurs on March 20, was occurring about 10 days later on the calendar. This had particular significance for the Catholic Church, which celebrated Easter on the first full moon after the equinox (fixed by the church at March 21), and set other religious dates depending on the date of Easter. “An error in the equinox thus introduced numerous errors in the entire religious calendar,” explains The Galileo Project.

Christopher Clavius, a Jesuit mathematician and astronomer, led a commission that proposed the Julian calendar be changed to better approximate the solar year. The solution was to drop a leap day three times in a 400-year cycle; every year divisible by 100, but not by 400, would lose its scheduled leap day. The result was a 365.2425-day year, nearly identical to the average solar year of 365.2424 days.

In order to restore March 20 or 21 as the day of the lunar equinox, Clavius recommended that 10 days be skipped. Pope Gregory XIII issued an order that Oct. 5-Oct. 14 be dropped from the calendar in 1582.

The Catholic countries in Europe adopted the order immediately, while most Protestant adopted in years later. Russia, Greece, and numerous other countries in Eastern Europe continued using the Julian calendar into the 20th century.

The modern Gregorian calendar can be traced back to the Roman calendar, which originally contained 10 now-familiar months (March-December) alternating between 29 and 30 days to mark the lunar movements. Two months, January and February, were later added to raise the number of days to 355. The missing 10 1/4 days were added through additional months every few years.

In the first century B.C., Julius Caesar ordered that the Roman calendar be amended. Under the Julian calendar, the lunar movements would no longer be considered; months would alternate between 30 and 31 days, except for February, which would have 29. According to legend, Augustus ordered that August, which had 30 days, be given an extra day taken from February. The months of September through December were adjusted, leading to the lengths of months that exist today.

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The difference between the Julian and Gregorian calendars is responsible for differences in some important historical dates. For example, because Britain and its colonies did not adopt the Gregorian calendar until 1752 (eliminating Sept. 3–13), there are multiple birth dates for many U.S. founding fathers; George Washington’s birthday is listed as Feb. 11 (Julian) and Feb. 22 (Gregorian).

Russia didn’t adopt the Gregorian calendar until 1919, meaning that the two parts of the 1919 Russian Revolution are known as the February Revolution and the October Revolution even though they occurred in March and November, respectively, on the Gregorian calendar.